Dell OptiPlex info on Wikipedia
box: midi tower
cpu: Intel Pentium III @ 733 MHz, max 2 cpus (must be equal, 2nd cpu needs VRM module)
memory: 2 slots, type RIMM, 512 MB (2 modules of 256 MB, max 1 GB)
disk: ad4: 190782MB <Seagate ST3200822AS 3.01> at ata2-master SATA150
optical drive: acd0: CDRW <Hewlett-Packard CD-Writer Plus 9500/1.0e> at ata1-master UDMA33
floppy drive: 3.5 inch 1..44 MB drive (A:, fd0)
floppy drive: 5.25 inch 1.2 MB drive (B:, fd1)
graphics card: nVidia Riva TNT2 Model 64
sound: snd_ich - pcm0: <Intel ICH (82801AA)> port 0xd800-0xd8ff,0xdc80-0xdcbf irq 17 at device 31.5 on pci0
pcm0: [ITHREAD]
pcm0: <Analog Devices AD1881 AC97 Codec>
network - wired: xl0: <3Com 3c905C-TX Fast Etherlink XL> port 0xec00-0xec7f mem 0xfafff800-0xfafff87f irq 18 at device 12.0 on pci2
miibus0: <MII bus> on xl0
xlphy0: <3c905C 10/100 internal PHY> PHY 24 on miibus0
xlphy0:  10baseT, 10baseT-FDX, 100baseTX, 100baseTX-FDX, auto
usb: 2 ports, only usb 1.1
boot: boot from network is supported
bios: version A10

More info: FreeBSD, OpenSolaris, Xubuntu on GX300.

event log:
2011-09-25: the machine would not boot; no output on the display at all. No error codes, no beeps. The battery (CR2032) was down, so I replaced that. Still no display output. After re-seating he CPU, the "dummy board" in the second cpu slot, the RDIMMs and the power connectors the machine is alive again.

2009-10-22: I replaced the failing drive. The new ad4 is a 500GB Western Digital (WDC WD5000AAKS-00TMA0 12.01C01), taken from an external WD elemts drive. Note: this drive has been getting too hot earlier, better install smartmontools asap.

2009-10-20: the ad4 hard drive (ST3200822AS) is failing. REad errors all over.
2009-10-20: added a 5.25 inch floppy drive (1.2 MB half height) as drive B:

2009-04-12: upgraded bios from version A07 to version A10 (latest available).

2009-04-08: I added a Silicon Image sil3114 SATA150 4 port SATA raid card (I'm not using the raid features). From dmesg:
atapci0: <SiI SiI 3114 SATA150 controller> port 0xecf8-0xecff,0xecf0-0xecf3,0xece0-0xece7,0xecd8-0xecdb,0xecc0-0xeccf mem 0xfafffc00-0xfaffffff irq 17 at device 8.0 on pci2
atapci0: [ITHREAD]
ata2: <ATA channel 0> on atapci0
ata2: [ITHREAD]
ata3: <ATA channel 1> on atapci0
ata3: [ITHREAD]
ata4: <ATA channel 2> on atapci0
ata4: [ITHREAD]
ata5: <ATA channel 3> on atapci0
ata5: [ITHREAD]
I also added a sata hard drive: ad4: 190782MB <Seagate ST3200822AS 3.01> at ata2-master SATA150

2009-03-12: I got the machine from a friend.